The institution of highly active antiretroviral therapy (HAART) has resulted in a major reduction of virus loads in individuals tolerating the regimen, a stabilization of the clinical course, and a significant decline in mortality/morbidity. Nonetheless, currently available combination antiretroviral therapy fails to eliminate HIV-1 from infected persons indicating the existence of a refractory reservoir(s) of virus in these individuals. During the past few years, the persistence of latent HIV-1 in resting CD4+ memory T lymphocytes has received considerable attention. It is currently thought that the virus which invariably emerges following the cessation of HAART emanates from this source. However, the clinical significance of this virus reservoir has been recently questioned by reports indicating: 1) a lack of correlation between the frequency of resting CD4+ T cells carrying inducible virus and the kinetics of the HIV-1 appearing in plasma following cessation of therapy and; 2) genotypic discordance between the HIV-1 resting memory cell pool and the rebounding viral RNA. These discrepancies suggest that resting CD4+ T cells may not be the principal source of the emerging HIV-1 and that other reservoirs of virus, such as tissue macrophage, must be seriously entertained. We have isolated and characterized a highly pathogenic SIV/HIV chimeric virus (SHIV), SHIV-DH12R, which induces a systemic depletion of CD4+ T lymphocytes in rhesus monkeys during the initial 3 to 4 weeks of infection. Nonetheless, high levels of viral RNA production continue unabated for an additional 2 to 5 months. In situ hybridization and immunohistochemical analyses revealed that tissue macrophage in lymph nodes, spleen, GI tract, liver, lung, brain, and kidney sustain high plasma virus loads in the absence of CD4+ T cells. Quantitative confocal immunofluorescence analysis indicated that greater than 95% of the virus producing cells in these tissues are macrophage and less than 2% are T lymphocytes. Interestingly, the administration of a potent reverse transcriptase inhibitor blocked virus production during the early T cell phase but not during the later macrophage phase of the SHIV-DH12R infection. When interpreted in the context of HIV-1 infections, these results implicate tissue macrophage as an important reservoir of virus in vivo. They become infected during the acute infection, gradually increase in number over time, and can be a major contributor to total body virus burden during the symptomatic phase of the human infection